1. Field of the Invention
The present invention relates to apparatus and methods of connecting tubular members in a wellbore.
2. Description of the Related Art
Oil and gas wells are completed by forming a borehole in the earth and then lining the borehole with a steel casing to form a wellbore. Typically, a number of sections of casing are used. A first section of casing is lowered into the wellbore and hung from the surface after the well has been drilled to a first designated depth. Cement is then circulated in the annulus between the outer wall of the casing and the borehole. The well is then drilled to a second designated depth and a second section of casing having a smaller diameter is run into the well. The second section may either be “hung off” in a wellhead at surface or is set at a depth such that the upper portion of the second section overlaps the lower portion of the first section of casing. If, in this second example, the casing does not extend to surface then the casing is referred to as a liner. The liner section is then fixed to the first section, such as by using a liner hanger. The second casing section or liner is then cemented. This process is typically repeated with additional casing sections of decreasing diameter until the well has been drilled to the total required depth.
The area above the production zone of the well is typically sealed using packers inside the casing or liner and connected to the surface via smaller diameter tubing. This provides a redundant barrier to leaks, and allows damaged sections to be replaced. Also, the smaller diameter of the tubing increases the velocity of the oil and gas. The natural pressure of the subsurface reservoir may be high enough for the oil or gas to flow to the surface. When this is not sufficient, such as for older wells, installing smaller diameter tubing may help the production, but artificial lift methods, such as gas lift, may also be needed. The well needs to be configured to receive the artificial lift apparatus.
In another well completion scheme it may be necessary to connect the liner string back to the surface (or a point higher up in the well). A string of tubing is then connected to the top of the liner section. In this manner, the casing section is sealingly “tied back” to the surface (or a point higher in the well).
Known methods for connecting a string of tubing into a downhole liner section typically involve the use of a tool known as a polished bore receptacle (PBR). The PBR is a separate tool which is screwed to the top of the liner section. The PBR has a smoothed cylindrical inner bore configured to receive the lower end of the tieback tubing. The tubing is landed in the PBR to form a sealed connection between the tubing and the liner. The lower portion of the tubing is configured with seals on its outer diameter and these seals seal within the PBR.
However, the majority of the length of the PBR is exposed and is susceptible to damage as other downhole tools are run into the wellbore. A downhole tool being run through the PBR may impact the polished surface of the PBR on its way downhole. This can cause damage that reduces the sealing ability of the PBR. Also, drilling debris can degrade the PBR sealing surfaces. In addition, it is known that associated components, such as tie back stingers and packers can leak, particularly in harsh environments.
The PBR allows for thermal expansion and contraction of the tieback liner, during which the liner seals can move up and down in the PBR. Over time this movement can cause the seals to wear and ultimately to fail. This is regarded to be one of the major limitations of a conventional PBR.
It is desirable to provide an alternative means of connecting to a lower section of liner which eliminates the need for a PBR or which reduces the likelihood of damage to a PBR. It is desirable to provide an alternative means of connecting to a lower section of liner which can eliminate the need for one or more associated components, such as anchors, tie back stingers and packers. It is desirable to provide an alternative means for providing a metal to metal seal between two tubular sections.
It is known to provide tools which cause the expansion of tubular sections in situ to connect the sections and provide a seal. These tools can include radially expandable members which, using fluid pressure, are urged outward radially into contact with a tubular section. When sufficient pressure is generated, the tubular section is expanded as it elastically and then plastically deforms.
There are a number of advantages of this over a conventional PBR. The tubing is anchored with sufficient resistance to the thermally generated axial loads. There is therefore little or no movement and so no wear. Also, this device has a metal to metal seal and so no elastomers to wear out. Also, the internal diameter of the device is not a polished seal surface and so its performance is much less affected by damage. Also, higher burst and collapse loads can be achieved.